1. Field of the Invention
This invention concerns tension mask color cathode ray tubes, and more particularly, relates to an improved front assembly having a mask with an aperture pattern which reduces beamlet degrouping errors.
2. Definitions
As used herein, the term "screen" refers to the field of discrete phosphor deposits on the inner surface of the tube faceplate which emit red, green or blue light upon excitation by the electron beams.
As used herein, the term "shadow mask" is a component of a color cathode ray tube located in spaced adjacency to the faceplate, one having a plurality of apertures for the passage of the electron beams that excite phosphors disposed on the screen of the faceplate. The shadow mask "shadows" the triads of phosphor deposits on the faceplate so that only the proper beam falls upon the assigned ones of the phosphor deposits. The shadow mask is also eeferred to as a "color selection electrode", or "parallax barrier." The shadow mask that is the subject of this invention is a flat, or "planar" mask.
As used herein, the term "pitch" means the center-to-center distance between shadow mask apertures. The symbol "Ph" refers to the horizontal pitch, or distance, between aperture centers, and the symbol "Pv" refers to the vertical pitch between aperture centers. Pho and Pvo are, respectively, the horizontal and vertical pitch of the mask apertures at the mask center.
As used herein, the term "grade" or "graded pitch" or "graded aperture dimension" means a shadow mask in which the pitch and/or aperture dimensions vary from one area of the mask to another; e.g. from the center of the mask to its periphery.
As used herein, "electron beamlet" means the portion of an electron beam passing through a mask aperture.
As used herein, the term "degrouping" refers to a a non-symmetrical placement of the beamlets of a deflected beamlet trio as the trio intercepts the screen. Degrouping error refers to the magnitude of the degrouping-induced misregistration of the beamlets relative to the impinged phosphor deposits. As a result of degrouping, a part or all of an outer beamlet or beamlets may fail to land on the assigned phosphor deposit(s) with consequent color impurities and reduced brightness in peripheral areas of the screen.
As used herein, the term "beam landing area" is that area of the screen upon which a beamlet falls.
As used herein, the term "positive guard band" means a condition wherein the beam landing area is smaller than the phosphor element upon which it lands; as a result, the area of the phosphor element unexcited by the beam serves as a positive guard band. The term "negative guard band" means a condition in which the beam landing area is larger than the phosphor element upon which it lands by a predetermined guard band area. In negative guard band screens, the margin of safety, or guard band, that prevents color impurities is conventionally covered with a light-absorbing material.